When scheduling the High Speed Downlink Shared Channel (HS-DSCH) in a WCDMA system, a number of user terminals are selected to receive data from a radio base station in a transport block that is transmitted in a time interval known as a subframe. The scheduling is based, among other things, on the Channel Quality Indicator (CQI), signalled from the user terminals to the radio base stations. The CQI is used to determine a Transport Format Resource Combination (TFRC), which in turn specifies a Transport Block Size (TBS), a modulation method and a set of HS-PDSCH codes, as well as the required transmission power in order to achieve a certain target Block Error Rate (BLER) for the transmission. A typical target BLER is 10%.
If, for example, the user terminal is overestimating the channel quality, the experienced BLER will be higher than the target BLER, and vice versa. Erroneous estimation will result in offsets in the CQI signalled from the user terminal. In order to adjust for such offsets in the signalled CQI, a CQI adjustment function may be used. This function translates the received CQI to a channel quality measure, which in turn can be adjusted up or down to compensate for negative or positive bias. The CQI adjustment function is based on a comparison of the received ACK/NACK ratio, that is, the experienced BLER, to the target BLER, and the received CQI is adjusted before it is used to select a TFRC so that the experienced BLER matches the target BLER
In some cases, in particular in low load scenarios, the target BLER may be achieved for all scheduled user terminals without using all the available power in the cell.